The objective assessment of the visual field using multi-focal stimulation has been reported recently (refs 1-7). Using different types of multifocal stimulus presentation (Sutter U.S. Pat. No. 4,846,567; Malov, International Patent Application No PCT/AU00/01483; and refs 14-17, the disclosures of which are hereby being incorporated by reference), stimulation of a large number of locations of the visual field can be performed simultaneously. Visually evoked cortical potentials (VEP) and electroretinograms (ERG) can be recorded from all areas of the field. For the VEP various electrode placements have been used. The best representation of the visual field was reported by the inventors with multichannel bipolar recordings (Klistorner and Graham, International Patent Application No. PCT/AU99/00340). Multifocal ERG recording has been performed with various electrodes (gold foil, DTL, Burian-Allen, gold lens). Good correlation was reported between the multifocal VEP and visual field loss in glaucoma (refs 2,5-7), and between the multifocal ERG and local retinal disease (ref 8), but not between the multifocal ERG and glaucoma (ref 9,10).
However, these recordings require a high resolution, large screen display (22 inch or larger), and subjects are required to sit close to the screen. The distance of the subject from the screen changes the area of field stimulated, and also changes the focal length and thus the required spectacle correction, so must be closely controlled during the recording. The CRT monitor also produces a large electromagnetic field which may affect the recordings when the subject is in close proximity to the screen. Recording is limited to one eye at the time, whereas with goggles it is possible to present a different stimulus to the two eyes at the same time. Therefore the concept of using a head mounted display provides a solution to these problems, and saves significantly on space requirements. It also allows for portability of the system. Binocular simultaneous multifocal recording reduces the recording time up to 50% by allowing two eyes to be tested simultaneously using different stimulus sequences for the two eyes.
A significant problem with multifocal VEP recordings has been the large inter-individual variability seen among the normal population, which limits the sensitivity of applying values from a normal data base when looking for small changes early in the disease process. A scaling algorithm has previously been reported by us (Klistomer & Graham, International patent application No. PCT/AU99/00340) which helped to reduce this variability. However, the scaling of VEP amplitudes according to background electroencephalogram levels as described in this patent has been found to be a superior technique for reducing inter-individual variability and increasing the sensitivity of the test.